This application is related to and claims priority from a British Patent Application No. 9813290.5 filed Jun. 20, 1998, and entitled xe2x80x9cA METHOD OF SYNTHESIZING AN AUDIO SIGNALxe2x80x9d, the entirety of which is explicitly incorporated herein by reference.
This invention relates to a method of synthesizing an audio signal having left and right channels corresponding to a virtual sound source at a given apparent location in space relative to a preferred position of a listener in use, the information in the channels including cues for perception of the direction of said virtual sound source from said preferred position.
The processing of audio signals to reproduce a three dimensional sound-field on replay to a listener having two ears has been a goal for inventors since the invention of stereo by Alan Blumlein in the 1930""s. One approach has been to use many sound reproduction channels to surround the listener with a multiplicity of sound sources such as loudspeakers. Another approach has been to use a dummy head having microphones positioned in the auditory canals of artificial ears to make sound recordings for headphone listening. An especially promising approach to the binaural synthesis of such a sound-field has been described in EP-B-0689756, which describes the synthesis of a sound-field using a pair of loudspeakers and only two signal channels, the sound-field nevertheless having directional information allowing a listener to perceive sound sources appearing to lie anywhere on a sphere surrounding the head of a listener placed at the centre of the sphere.
A drawback with such systems developed in the past has been that although the recreated sound-field has directional information, it has been difficult to recreate the perception of having a sound source which is perceived to move towards or away from a listener with time, or that of a physically large sound source.
According to a first aspect of the invention there is provided a method as specified in claims 1-11. According to a second aspect of the invention there is provided apparatus as specified in claim 12. According to a third aspect of the invention there is provided an audio signal as specified in claim 13.
It might be argued that to synthesize a large area sound source one might use a large area source for a particular HRTF measurement. However, if a large loudspeaker is used for the HRTF measurements, then the results are gross and imprecise. The measured HRTF amplitude characteristics become meaningless, because they are effectively the averaged surmmation of many. In addition, it becomes impossible to determine a precise value for the inter-aural time-delay element of the HRTF (FIG. 1), which is a critical parameter. The results are therefore spatially vague, and cannot be used to create distinctly distinguishable virtual sources.